Field of the Invention
This invention relates to a transmissive screen for use in a rear projection display device such as a rear projection television and to a light diffusing plate for use in such a transmissive screen.
A rear projection television generally uses a transmissive screen having a Fresnel lens, a lenticular lens and a light diffusing plate containing particles of a light diffusing material dispersed therein. The Fresnel lens serves to collect light projected from a light source and to direct it toward a viewer. The lenticular lens serves to distribute light within a region where the viewer can see and to use light effectively. The light diffusing plate has a function of diffusing projected light to produce clear images.
In general, a rear projection television is required to have a wider viewing angle horizontally than vertically. Thus, a transmissive screen for use in a projection television has a light diffusing plate having a viewer side surface on which a lenticular lens for horizontal diffusion is laminated.
The horizontal viewing angle of a transmissive screen depends mainly on the performance of the lenticular lens for horizontal diffusion, The vertical viewing angle of a transmissive screen is determined by a light diffusing material contained in the light diffusing plate. The vertical viewing angle determined by the light diffusing material is much narrower than the horizontal viewing angle determined by a lenticular lens for horizontal diffusion. Thus, the conventional rear projection display device has a problem, because luminance uniformity of the screen cannot be satisfactorily achieved and a shading phenomenon explained below is apt to occur.
As shown in FIG. 11(a), when light from a light image source 102 is projected on a screen 101 and when viewers P and Q see the screen 101, the luminance distribution on the screen 101 for the viewers P and Q are as shown in FIG. 11(b) and FIG. 11(c), respectively.
Namely, when the viewer P, who is in right front of the screen 101, sees the screen 101, the bottom area A and the top area C of the screen 101 has lower luminance and look darker to the viewer P than the middle area B thereof does as shown in FIG. 11(b) since the bottom area A and the top area C of the screen 101 are farther away from the viewer P than the middle area B is. When the viewer Q, who is not in right front of the screen 101, sees the screen 101, the top area C of the screen 101, which is closest to the viewer Q, has highest luminance and looks brightest to the viewer Q, and the areas of the screen 101 which are farther away from the viewer Q have lower luminance and look darker to the viewer Q. Especially, since the light diffusion angle of the screen 101 is small in the vertical direction, a shading phenomenon is likely to occur, i.e. luminance uniformity tends to be insufficient in the bottom area A of the screen.
To cope with the shading problem, a light diffusing plate having a lenticular lens or a Fresnel lens in which the light diffusing ability of the top and bottom areas is higher than that of the middle area has been proposed. For example, JP-A-H07-134338 discloses a screen using a lenticular lens for horizontal diffusion and/or a lenticular lens for vertical diffusion in which the diffusion angle of the lens units in the peripheral area is larger than the diffusion angle of the lens units in the center area. JP-A-H08-334837 discloses a screen using a light diffusing plate having a surface in which a lenticular lens for vertical diffusion is directly formed. The heights of the lens units of the lenticular lens and the intervals therebetween are adjusted so that the light diffusing ability of the top and bottom areas of the screen can be higher than that of the middle area thereof. JP-A-2001-228548 discloses a screen using a light diffusing plate having a light diffusing layer which is thicker at the top and bottom than at the middle so that the light diffusion angle at the top and bottom areas of the screen can be larger than that at the middle area thereof.
In the screens disclosed in JP-A-H07-134338 and JP-A-H08-334837, the peripheral area of the lenticular lens has a surface shape which is different from that of the middle area thereof so that the light diffusing ability of the peripheral area of the screen can be enhanced. To produce a lenticular lens of desired size and shape, the surface shape of the lens must be precisely controlled. It is, however, difficult to design and produce a carving roll or a mold for producing such a lens. Thus, it is difficult to produce such a lens stably.
When the thickness of the top and bottom of a light diffusing layer is increased to enhance the light diffusing ability of the top and bottom areas of a screen so that the vertical viewing angle of the screen can be widened as disclosed in JP-A-2001-228548, the following problem occurs. Thus, when a light diffusing material with a low light transmittance is used and the light diffusing layer is formed such that the top and bottom areas of the light diffusing layer have sufficient light diffusing ability, the top and bottom areas of the light diffusing layer have such a low light transmittance and a low luminance that a shading phenomenon occurs in the top and bottom areas of the screen. When a light diffusing material with a high light transmittance is used and the light diffusing layer is formed such that the top and bottom areas of the light diffusing layer have sufficient light diffusing ability, the middle area of the light diffusing layer becomes thin. Then, the middle area of the screen has an excessively high luminance, and a scintillation phenomenon (glare of the screen) or a hot band phenomenon (a phenomenon in which a significantly bright area appears on the screen) occurs in the middle area of the screen.
Thus, with the prior art screens, the light diffusing ability of the both end areas of a light diffusing plate in the vertical or horizontal direction cannot be made higher than that of the middle area thereof without decreasing the light transmittance of the both ends areas of the light diffusing plate or without excessively increasing the light transmittance of the middle area thereof.